In order to prevent the over-discharge or overcharge of a secondary battery, the state of charge (SOC) as the charge state value is estimated. For estimating SOC, a current integrating method of momentarily integrating the charge current value and discharge current value of the secondary battery is employed. Furthermore, Patent Literature 1 discloses the following charge state estimation device of a secondary battery. The charge state estimation device has a condition where, in order to stabilize the product of the internal resistance and the current, the limit current is increased when the internal resistance is low and the temperature of the secondary battery is high. Under this condition, the charge state estimation device assumes that the secondary battery is stable when the charge/discharge current is the limit current or less, and sets the inter-terminal voltage in the stable state as the open circuit voltage (OCV).
Patent Literature 2 discloses an estimating means of a battery charge state and a battery degradation estimating method. In the means and method, it is taken into consideration that the current-voltage characteristic of a secondary battery has a large hysteresis and the internal resistance increases with the degradation. In this disclosure, the SOC estimated by the current integrating method is set as a pseudo SOC, an estimated OCV corresponding to the pseudo SOC is denoted with Voc, and Voc is added to a dynamic voltage variable component Vdyn considering the hysteresis and a voltage component Vr due to the internal resistance, thereby providing (Voc+Vdyn+Vr). The pseudo SOC is corrected to the SOC corresponding to this equation.